Anne Fontaine's film about two lifelong friends who fall for each other's sons is all vapidly beautiful surface, an impeccably tasteful picture about some awfully tasteless decisions.

A ludicrous melodrama that begs to be handled as an over-the-top sex farce is instead treated with the solemnity of a wake, albeit one with a rather lenient dress code, in “Two Mothers.” Fully embracing the narcissism and misplaced priorities of its four hopelessly inseparable characters, Anne Fontaine’s film about two lifelong friends who fall for each other’s sons is all vapidly beautiful surface, an impeccably tasteful picture about some awfully tasteless decisions. Typically classy performances by Naomi Watts and Robin Wright lend the material more dignity and interest than it warrants, spelling lucrative inroads with distaff audiences in arthouse play.

An opening sequence swiftly establishes the schematic symmetries at the core of this Australia-set adaptation of Doris Lessing’s novella “The Grandmothers.” Best friends since they were growing up along the coast of New South Wales, Roz (Wright) and Lil (Watts) have a lot in common: They’re both strikingly beautiful blondes who live in adjacent beachfront houses, and each one has a son of about 20. Roz’s boy, Tom (James Frecheville), and Lil’s son, Ian (Xavier Samuel), are also best friends and surfing buddies. With Lil a longtime widow and Roz’s husband (Ben Mendelsohn) away in Sydney on business, the four love to idle away their free time together with wine, cards and occasional dancing.

It’s all a bit more self-involved than what most would consider healthy, and sure enough, it’s not long before Ian quietly makes a move on Roz, who puts up little resistance. Tom, having witnessed the encounter, marches over to Lil’s pad the next morning and seduces her in almost retaliatory fashion. Common sense and decency momentarily intrude, but soon the characters decide they rather like this arrangement and can make it work, giving the matter about as much time or consideration as it took to write this sentence. Or, as Lil puts it: “I don’t want to stop. I don’t see why we have to.”

Perhaps it takes a Gallic sensibility to suggest that this unconventional design for living could be a viable one, and there are moments when Fontaine (“Coco Before Chanel”), a French director making her English-language debut, seems prepared to tap into the material’s vulgar comic potential. Yet the laughs that are generated seem mostly unintentional, as this softcore cougar fantasy proceeds with the hushed solemnity of a Bergman chamber drama.

Despite their self-serious approach, Fontaine and scribe Christopher Hampton (usually a dab hand at tony literary material) tend to usher their characters away from the tough, juicy confrontations that would provide the requisite payoffs and presumably enable audience understanding. Exquisite beauty and decorum, as exemplified by Christophe Beaucarne’s pristine widescreen images and Christopher Gordon’s lush orchestrations, are apparently all the explanation or justification one needs. Not only do Lil and Roz fail to give each other a good throttling; they barely even raise their voices, treating each other instead with simpering politeness.

As far as Ian and Tom go, it’s unclear what they see in their significant mothers beyond physical attraction. Frankly, it’s unclear what these strapping Adonises think of anything; far more puzzling than the film’s May-December content is the revelation that Ian, played as a hulking, incommunicative slab by Frecheville, aspires to be a theater director.

Roz’s marriage crumbles and two years pass, during which this menage a quatre more or less retreats from the knowledge and judgment of the outside world and into a perpetual state of beach-bum bliss. Eventually, however, Tom’s wandering eye and the sudden appearance of wrinkles on Lil’s face determine that the situation is unsustainable, and the film’s second half sees the boys moving on and eventually starting their own families even as the pain of separation persists. (Tom and Ian each have a daughter, perhaps laying the groundwork for a much ickier sequel.)

Watts and Wright can both convey nuance and quiet intelligence even when seeming to do nothing in particular, and it’s lovely to see them share the screen at length, even if they’re largely coasting on serene presence in lieu of meaty material. A bit of a blank at first, Samuel gradually comes into his own as Ian emerges the most hotheaded and honest member of the foursome, achieving one of the few moments when real feelings and emotional stakes seem to breach the film’s immaculate surface. In all other respects, “Two Mothers” paints a picture of privileged isolation so stilted, otherworldly and ultimately impenetrable, it almost qualifies as science fiction.